add placeholders at internal host

**Is your feature request related to a problem? Please describe.**
My app is userless, so I use authentik-proxy for authenticating my user.
To enable multiple users i like to deploy multiple instances for the app - one for each user, e.g.

http://app-foo/
http://app-bar/
http://app-john/

But right now i can only forward an authenticated user to the same _internal host_.

**Describe the solution you'd like**
If i could make the _internal host_ somewhat dynamic by adding a placeholder for the username i could proxy every user to its own application instance. e.g.:

```
http://app-{preferred_username}/
```

**Describe alternatives you've considered**
none

**Additional context**
I am using authentik-proxy to authenticate an authless app. Now while this works for authentication I am struggling with identification.

Thanks in advance

cheers

The request concerns authentik-proxy's internal host setting and a username placeholder; no source file or test is identified. Start by locating the configuration and request-routing entry points for the internal host, then check how user attributes are exposed and safely interpolated. Done means an authenticated user's preferred username selects the corresponding app instance, with coverage for the requested routing behavior.

Written by the indexing model from the issue text.